When designing within CreativeStudio, you have the option to make certain content, whether text or images, editable by the buyer. This can be especially useful for customizable content. A use-case scenario would be for mugs with customizable names. Now, any name can become available on your design without your needing to create a new design for each name.
To make text editable, you can click the check box on the text item to make editable in GO Expression:
This same option is available for images:
Buyers will then be able to customize the product prior to adding to their cart: